Systemic juvenile idiopathic arthritis (SJIA) is a rheumatic condition characterized by arthritis, quotidian fever, evanescent rash, and sometimes other types of systemic inflammation. SJIA is currently thought to be a multigenic, autoinflammatory disease, but the molecular details of its pathogenesis are still unknown. A striking feature of SJIA is its association with macrophage activation syndrome (MAS), a complication that can be fatal. This clinical aspect, together with data from immunophenotypic studies of SJIA, points to the monocyte lineage as being important in disease pathogenesis. Activated monocytes express components of the inflammasome, a multiprotein complex that senses infectious or noxious stimuli and activates caspase-1, leading to maturation and secretion of IL-1beta and IL-18 and sometimes to host cell death IL-1beta and IL-18 have been implicated in SJIA. Based on our preliminary data, we hypothesize that monocytes from SJIA patients harbor a defect in the inflammasome pathway. The objective of this R21 proposal is to discover host pathways and genes that influence IL- 1beta maturation and release by monocytes and to test these as candidate disease associated factors. In Aim 1, we will take a candidate approach to characterize possible defects in SJIA monocytes by measuring known molecular events related to IL-1beta secretion in cells from SJIA patients compared to cells from age-matched immunologically normal children. Specifically, we will measure cytokine release, activation of caspase-1, caspase-1 regulated cell death, levels of expression of Rab 39a, the purinergic receptor P2X7R, and the pore-forming protein, pannexin-1. Findings will be followed up with sequencing of associated candidate genes in 35 SJIA patients. In Aim 2, we will take a less biased approach and conduct a forward genetic screen to identify host factors that influence IL-1beta secretion in normal cells. This screen will likely provide additional candidate factors potentially affecting IL-1beta biology in SJIA. Our results will delineate molecular components regulating IL-1 processing and secretion and, ultimately, could result in new and innovative approaches to the prevention and treatment of SJIA and other autoinflammatory diseases.